doc/go1.18: mention runtime/pprof improvements

doc/go1.18: mention runtime/pprof improvements

For #47694.

Change-Id: Ib49145a58b8388d35267cf4b0caa730d7e436d06
Reviewed-on: https://go-review.googlesource.com/c/go/+/387574
Reviewed-by: Michael Pratt <mpratt@google.com>
Trust: Michael Pratt <mpratt@google.com>
Run-TryBot: Michael Pratt <mpratt@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Rhys Hiltner <rhys@justin.tv>
Reviewed-by: Bryan Mills <bcmills@google.com>
Trust: Bryan Mills <bcmills@google.com>